C A R B O N 6 7 ( 2 0 1 4 ) 1 4 0 – 1 4 5

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Erosion of carbon/carbon composites using alow-velocity, high-particle-concentrationtwo-phase jet in a solid rocket motor

0008-6223/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.carbon.2013.09.072

* Corresponding author: Fax: +86 (0)29 88494163.E-mail address: leeqiang@nwpu.edu.cn (Q. Li).

Qiang Li *, Jiang Li, Guo-qiang He, Pei-jin Liu

Science and Technology on Combustion, Internal Flow and Thermal-Structure Laboratory, Northwestern Polytechnical University, Xi’an,

Shaanxi 710072, PR China

A R T I C L E I N F O

Article history:

Received 24 June 2013

Accepted 25 September 2013

Available online 1 October 2013

A B S T R A C T

The erosion of a four-direction carbon/carbon composite test piece using a low-velocity,

high-particle-concentration two-phase jet was studied by the hot firing test of a small solid

rocket motor with an elaborately designed flow path. The linear ablation rates were mea-

sured. The ablation surface and microstructure of the carbon/carbon composites were

studied by scanning electron microscopy, and the ablation mechanism was investigated.

Within the parameter range studied in this paper, mechanical erosion is found to play a

dominant role in the ablation of carbon/carbon composites in the impact region. Moreover,

the effect of chemical ablation is weak. The erosion of carbon/carbon composites results in

blunt fracture tip fibers and a lamellar matrix. The particle impact mass flux is more dom-

inant than the particle impact velocity in the erosion of carbon/carbon composites. At

mesoscale, the carbon rods are more resistant to mechanical erosion than fiber bundles.

At microscale, the carbon fibers are more susceptible to mechanical erosion than the car-

bon matrix, whereas the carbon fibers are more resistant to chemical ablation than the

matrix and interface.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon/carbon (C/C) composites are one of the most ideal

thermal-protecting materials in aerospace applications be-

cause of the extraordinary and unique characteristics, such

as low density, high strength, remarkable thermal stability,

low thermal conductivity, and extremely low ablation rate

[1,2]. The ablation of C/C composites is complex. The process

involves the ablation environment, chemical ablation,

mechanical erosion, and structures of the composites [3,4].

In the last few years, the ablation performance of C/C com-

posites has been experimentally studied using plasma abla-

tion, oxyacetylene torch, arc heating and motor firing tests

[5,6]. Furthermore, the effects of combustion chamber

pressure and temperature, solid propellant type, combustion

product composition, as well as density and graphitization

degree of C/C composites on the ablation characterizations

of C/C composites have been examined. The ablation mecha-

nisms of C/C composites have also been investigated through

their morphology [7–10]. However, most of these studies have

focused on the ablation of the C/C composite nozzle throat

because of its importance. Few studies have been conducted

on the erosion of C/C composites by a low-velocity, high-par-

ticle-concentration two-phase jet, such as that the erosion of

the entrance and head cap of the submerged nozzle, the nose

tip of the submerged entry nozzle, and the entrance of the

convergent–divergent nozzle [11], where the gas velocity is

low and chemical ablation is weak. The shapes of these

C A R B O N 6 7 ( 2 0 1 4 ) 1 4 0 – 1 4 5 141

components significantly influence local gas flow and,

consequently, the ablation of the nozzle throat and the overall

performance of the solid rocket motor (SRM). Thus, the

erosion of C/C composites using a low-velocity, high-parti-

cle-concentration two-phase jet must be investigated.

In this study, an SRM with a specially designed flow path

was used to investigate the erosion behavior of 4D C/C com-

posites using a low-velocity, high-particle-concentration

two-phase jet in an SRM hot firing test. Ablation rates were

measured, and the ablation performance and dominant

factors were interpreted according to theoretical simulations.

The ablation morphology and microstructure of the C/C

composites were investigated, and the ablation mechanism

was discussed.

2. Experimental

2.1. Description of the ablative media

The C/C composites comprising the C/C composite test piece

studied in this paper are the same as those in Ref. [7]. A

schematic of the composite preform is shown in Fig. 1. T300

PAN-based carbon fibers and coal-tar pitch were used as the

reinforcement and the matrix, respectively. The diameter of

the carbon rod was controlled to be 1.5 mm, and the spacing

distance between carbon rods was fixed at 4.0 mm. The width

and thickness of the fiber bundles were 2.0 and 0.6 mm,

respectively. The fiber bundles were preformed into hexago-

nal shapes with three directional axes (W, X, and Y) aligned

along the same plane at 120�. The carbon rods were fixed

perpendicular to the bundles. The apparent density of the

as-prepared composites was 1.93–1.94 g/cm3. Details of the

heat treatment of C/C composites can be found in Ref. [7]

and the references therein.

2.2. Ablation test

An SRM hot firing system was utilized to investigate the

erosion of C/C composites using a low-velocity, high-parti-

cle-concentration two-phase jet. The hot firing test system

primarily consisted of combustion chamber, solid propellant

coated by inhibitor layer, steel case, pressure gauge, jet

controller, test section, C/C composite test piece and nozzle,

and so on, as shown in Fig. 2. The jet controller was

connected to the combustion chamber with an integral type

flange, and the rectangular test section was linked to the

Fig. 1 – Schematic of the composite preform. (A color version

of this figure can be viewed online.)

circular jet controller with an angle joint. The C/C composite

test piece was mounted onto the bottom (inner) surface of the

test section. The angle formed by the axis of the jet controller

and the center line of the test section was adjustable.

In the hot firing test, the two-phase flow released from the

propellant burning surface flowed through the jet controller,

in which the particle number density of the two-phase jet

was increased and the spatial distribution was homogenized.

Upon leaving the exit of the jet controller, the combustion

gases simultaneously changed in flow direction. Under the

action of inertia, most condensed particles formed a

low-velocity, high-particle-concentration two-phase jet that

impacts the surface of the C/C composite test piece. Subse-

quently, erosion occurred.

The Z-direction of the preform was perpendicular to the

top surface of the C/C composite test piece, and the WXY-

plane was parallel to the top surface of the C/C composite test

piece. The SRM was loaded with a composite propellant

comprising 66.2 wt.% ammonium perchlorate, 15.4 wt.%

hydroxyl-terminated polybutadiene, 17.5 wt.% aluminum

powder, and other additives. In the hot firing test, the end-

burning grain was used in the SRM to maintain constant

chamber pressure. The angle formed by the axis of the jet

controller and the center line of the test section was fixed at

30� in the hot firing test.

2.3. Characterization

Before and after the hot firing test, the profiles of the C/C com-

posite test piece were obtained using a surface profile mea-

suring instrument (Proscan 2100, Scantron).

The microstructure and morphology of the ablation sur-

face of the C/C composites were characterized by scanning

electron microscopy (SEM; JEOL, JSM-6390A) and energy-dis-

persive spectroscopy (EDS).

3. Results and discussion

3.1. Ablation environment

In an SRM, the combustion products of the metalized propel-

lant provide an ablation environment for the C/C composites

and determine ablation behavior.

In this paper, the equilibrium compositions of the propel-

lant combustion products were calculated by the Chemical

Equilibrium with Application Code, a free software for

calculating chemical equilibrium products based on the

Fig. 2 – Schematic of the hot firing test motor.

Table 1 – Chemical compositions of SRM combustionproducts in the combustion chamber.

Species Mole fraction (mol%)

H2 (g) 33.158CO (g) 25.682HCl (g) 12.729H2O (g) 8.583CO2 (g) 8.360N2 (g) 7.204Al2O3 (g) 7.219

Fig. 3 – Distributions of two-phase flow parameters.

142 C A R B O N 6 7 ( 2 0 1 4 ) 1 4 0 – 1 4 5

free-energy-minimization principle [12,13]. The estimated

chamber pressure (7.0 MPa) was used to calculate chemical

equilibrium. Results show that the combustion products in

the combustion chamber comprise various species, as shown

in Table 1. Other parameters such as stagnation pressure,

stagnation temperature, and mass fraction of aluminum

oxide in the combustion products are shown in Table 2.

A three-dimensional model of the viscous compressible

two-phase flow in the rocket motor was established. Fluent

Software (version 6.3) was used for the computation using

Reynolds-averaged Navier–Stokes equations with extra

source terms representing the interactions between the two

phases. The Lagrangian method was used to track the con-

densed particles. The Reynolds number was evaluated to be

approximately 57 000. Spalart–Allmaras one-equation turbu-

lence model, which was satisfactorily accurate for transonic

compressible flow, was used. Gambit was used to generate a

mesh containing 682 000 tetrahedral cells. The mass fluxes

and temperatures of both phases were specified at the inlet

and diameter distributions of the condensed particles. No

boundary condition was necessary at the exit of the sub-

merged nozzle for supersonic flow. The measured sizes of

the condensed particles from the chamber of SRM [14] were

used as a boundary condition during simulation. Collisions

between particles were neglected, and the impact of con-

densed particles on the surface of C/C composite test piece

was regarded as perfectly elastic and specular reflection.

The parameters of condensed particles impacting on the

surface of the C/C composite test piece were counted and

post-processed with user-defined functions. In this paper,

the particle impact mass flux is defined as the total mass of

particles impacting the surface of the C/C composite test

piece per unit area per unit time. The particle impact velocity

is defined as the number-averaged velocities of particles

impacting the same boundary surface triangular mesh at-

tached onto the top surface of the C/C composite test piece.

The computed results are plotted in Fig. 3, along with the

gas velocity extracted from the symmetry plane of the test

section, where x is the distance in the X-direction from the

forefront of the C/C composite test piece (x = 0.330 m). The

Table 2 – Combustion product parameters in thecombustion chamber of the test SRM.

Stagnation pressure (MPa) 7.0Stagnation temperature (K) 3530.0Al2O3(l) (wt.%) 31.1

particle impact mass flux has the maximum value, where

x = 0.3638 m, whereas both gas and particle impact velocities

have the maximum value, where x = 0.3671 m. The maximum

values of velocities lag behind those of impact mass flux in

space.

Fig. 4 shows the recorded time history of pressure in the

combustion chamber. The duration of the hot firing test is

about 6.2 s, and the maximum chamber pressure is about

6.8 MPa and lasts for approximately 4.2 s.

3.2. Macro-scale ablation rate

Fig. 5(a) shows the photograph of the C/C composite test piece

before the hot firing test. Before the test, the top surface of the

C/C composite test piece is rather rough. Although most of

the fiber bundles are damaged because of machining, the ar-

ray of carbon rods and bundles are visible and in good order,

the tips of the carbon rods are lower, and small shallow pores

are formed. Fig. 5(b) shows the photograph of the C/C com-

posite test piece after the hot firing test. The C/C composite

test piece is still rough. In the impact region of the two-phase

jet, the fiber bundles on the top layer are damaged and a small

cone-shaped pit is formed. Downstream of the impact region,

some large particles about 1.0 mm in diameter can be seen

sticking to the tips of the carbon rods.

The surface profile of the C/C composite test piece was

measured before and after the hot firing test. The spatial res-

olution of the measured data is 0.02 mm and then the data

Fig. 4 – Time history of combustion chamber pressure.

Fig. 6 – 2D recession map of the C/C composite surface.

C A R B O N 6 7 ( 2 0 1 4 ) 1 4 0 – 1 4 5 143

were averaged every 0.2 mm. Fig. 6 shows the 2D recession

map of the cross section, which lies along the X–Y plane

and passes through the bottom of the pit. The top diameter

of the pit is approximately 5.2 mm, and its maximum depth

approximately 0.53 mm. A close comparison shows that the

position of the bottom of the cone-shaped pit (x = 0.361–

0.364 m) is almost the same as the position where the particle

impact mass flux has the maximum value (x = 0.3638 m). This

finding indicates that, within the parameter range studied,

the particle impact mass flux is more dominant than the par-

ticle impact velocity in the erosion of the C/C composites.

The maximum ablation amount is found at the bottom of

the cone-shaped pit. A local magnification of the 2D recession

map shows that at the bottom of the pit, the tip of the carbon

rod is higher than the surrounding bundles at mesoscale,

with a height difference of about 0.122 mm. The measured

ablation rate of the carbon rod is about 0.067 mm/s, whereas

that of the bundles is approximately 0.089 mm/s. The ablation

rate is immeasurable at macroscale 20.0 mm away from the

cone-shaped pit.

3.3. Micro-scale ablation morphology

Fig. 7(a) shows the morphology of the post-test C/C composite

test piece. Near the impact region of the two-phase flow jet,

the ablated surface of the C/C composite test piece is rougher

at mesoscale. Numerous spherical Al2O3 particles with diam-

eters ranging from 100 to 300 lm are deposited onto the ab-

lated surface of the fiber bundles and the tips of carbon

rods. The fiber bundles are denudated and burned into

broom-like shapes. The tips of the carbon rods at the bottom

Fig. 5 – Photograph of the C/C composite test piece before

and after the hot firing test: (a) pre-test, and (b) post-test.

Fig. 7 – Morphology of the C/C composite test piece: (a) pit,

(b) carbon rod, and (c) morphology of the carbon rod

downstream of the pit.

Table 3 – EDS characterization of residuals in pore.

Element (keV) Mass (%) Error (%) Atom (%)

C 0.277 21.63 0.12 31.88O 0.525 37.14 0.14 41.09Al 1.486 40.78 0.10 26.75Si 1.735 0.49 0.16 0.28Total 100.00 100.00

144 C A R B O N 6 7 ( 2 0 1 4 ) 1 4 0 – 1 4 5

of the pit are coarser and higher than the surrounding fiber

bundles, indicating that the carbon rods are more resistant

to mechanical erosion than the bundles. The carbon rods

and fiber bundles outside the pit show no obvious damage

or ablation. The velocity of the two-phase flow is relatively

smaller than the velocity of the two-phase flow in the nozzle

throat. Consequently, the convective heat flux transferring

from combustion products to the solid is small, and the sur-

face temperature of the C/C composites is low. As a result,

the rate of heterogeneous reactions between the oxidizing

species present in the combustion products of the propellant

and C/C composites is lower [1,10], and chemical ablation is

weakened. Nevertheless, the maximum measured ablation

rate of the C/C composite test piece is comparable with that

of laboratory SRM experimental data [7–9]. This finding indi-

cates that near the impact region, mechanical erosion from

the impact of condensed particles on the top surface of the

C/C composite test piece plays a fully predominant role.

Moreover, the carbon rods are damaged and the fiber bundles

are denudated.

Fig. 7(b) shows the high-magnification morphology of the

carbon rod at the bottom of the pit shown in Fig. 7(a). A pore

is present at the tip of the carbon rod. EDS shows that the

residuals in the pore are mainly Al2O3 (Table 3), and it is de-

duced that pore formation is due to the impact and damage

of larger condensed particles. The fibers are peeled off from

the carbon matrix, and blunt fracture tips are formed. The

tips of the carbon fibers are lower than the matrix, indicating

that the carbon matrix is more resistant to mechanical ero-

sion than the carbon fibers. In design and manufacturing,

fine-weaving C/C composites with low-fiber-density carbon

rods should be used as constituents for the thermal protec-

tion components subjected to severe erosion of two-phase

flow.

Fig. 7(c) shows the high-magnification SEM image of the

carbon rod 40 mm downstream of the impact region where

the ablation rate is immeasurable at macroscale. The carbon

fibers are arranged in good order, and the tips of the fibers

are almost flat. These features show that the material has

only started its chemical ablation and has not had sufficient

time to develop a steady morphology. These results indicate

that the C/C composites are less ablated because of low gas

velocity, low particle impact velocity and small impact mass

flux in this local area. The irregularity of the surface is due

to the machining before the test. The interface, which can

be well resolved prior to testing, is burned out although the

surface temperature of the C/C composites may be lower than

that of the nozzle throat, ring-like gaps between the carbon

fibers and matrix are formed, suggesting that the interface

can be more prone to oxidization than the carbon matrix

and fibers. A lamellar matrix can be seen in the low-fiber-den-

sity area, whereas the matrix in the high-fiber-density area is

burned out and gaps are formed. This phenomenon indicates

that carbon fibers are more resistant to chemical ablation

than the matrix and interface. For the thermal protection

components subjected to severe chemical ablation, rods with

high carbon fiber density should be used.

3.4. Ablation analysis

The ablation of C/C composites is very complex and involves

both chemical ablation and mechanical erosion, which deter-

mine the ablation behavior of C/C composites.

Chemical ablation is widely known as the primary cause of

ablation and is referred to as the heterogeneous reactions

between the oxidizing species present in the combustion

products of the solid propellant and the C/C composites.

According to the classical Arrhenius law, the oxidation rate

varies severely with temperature and the oxidant partial pres-

sure [4,15]. Mechanical erosion from condensed particles is

regarded as another main factor contributing to the ablation

of C/C composites [6,7,9,10]. The role of the condensed parti-

cles in solid propellant combustion products can be classified

into two categories: mechanical force and mechanocaloric ef-

fect. In the hot firing test in this paper, the chemical ablation

is weakened due to the low velocity of combustion gases

flowing over the C/C composite test piece. Nevertheless, the

erosion of particles is predominant in the ablation of the C/

C composites.

A multiscale erosion model is necessary for the C/C com-

posites. Fig. 7(b) shows that the size of the particle impact

damage is comparable with the microstructural size scale of

the C/C composites. Consequently, the microstructural con-

straints must be considered and an ‘‘averaging law’’ is not

applicable at microscale, particle impact events occur in one

of the two phases, carbon fiber and matrix. At mesoscale,

the particle impact damage to carbon rod and bundle must

also be differentiated, as evidently shown in Fig. 7(a). To mod-

el the erosion process, a control volume or minimal periodic

unit should first be determined at both microscale and meso-

scale for the C/C composites, and a multiscale erosion model

is essential.

At microscale, when one particle impacts on the C/C

composites surface, the mechanical force between the parti-

cle and the C/C composites can be calculated by the Hertz-

ian equation for a pure elastic collision [16]. By assuming

that the contact region to be circular, the maximum contact

area and the duration of the impact have been derived by

C A R B O N 6 7 ( 2 0 1 4 ) 1 4 0 – 1 4 5 145

Hertzian and the maximum mechanical stress Fp can be

calculated. The probability that the particle impacts on the

carbon fiber or the matrix can be evaluated depending on

the structure of the microscale minimal periodic unit. When

the impact event occurs in the carbon fiber, the maximum

mechanical stress Fp can be decomposed into normal stress

Fn and tangential stress Ft, which are parallel and perpendic-

ular to the fiber orientation, respectively. When the normal

stress Fn or the tangential stress Ft exceeds the impact yield-

ing stress of the carbon fiber, erosion occurs. The erosion

can be classified into two different modes: brittle mode

and ductile mode, which are dominated by the normal

stress and the tangential stress, respectively. For the carbon

fiber, both modes would take place simultaneously and

should be considered. On erosion, the mass loss or volume

loss of the carbon fiber should be counted and the effect of

particle impact damage on fiber morphology must be deter-

mined and transferred to that at mesoscale. If the impact

event occurs in the matrix, the maximum mechanical stress

Fp can be decomposed into normal stress Fn and tangential

stress Ft, which are perpendicular and parallel to the matrix

surface, respectively. In the similar way, particle impact

damage can be evaluated.

At mesoscale, the microscale model is of great directive

significance. At macroscale, the linear or inverse rules of mix-

ture may be used to evaluate the erosion rate of the C/C

composites.

In the preceding description, many correlations must be

established and validated, such as the accurate evaluation

of the mechanical stress, the measurement of the intrinsic

parameters of the C/C composites at different scales, the ef-

fect of particle impact damage on morphologies of the two

phases, the mass loss or volume loss of the C/C composites

at different scales, the way the impact damage is transferred

from microscale to mesoscale and from mesoscale to macro-

scale and so on. Further investigations on these correlations

should be thoroughly carried out.

4. Conclusions

The erosion of a 4D C/C composite test piece by low-velocity,

high-particle-concentration two-phase jet was investigated

using an SRM. The average ablation rates were measured,

and the ablation performance, mechanism, and microstruc-

ture of the C/C composites were investigated. The effect of

chemical ablation is found to be weak due to low gas velocity.

The comparatively high ablation rates are mainly attributed

to mechanical erosion. At microscale, the carbon fibers are

more susceptible to mechanical erosion than the carbon ma-

trix, whereas carbon fibers are more resistant to chemical

ablation than the matrix and interface. At mesoscale, the car-

bon rods are more resistant to mechanical erosion than fiber

bundles. In applications, fine-weaving C/C composites with

low-fiber-density carbon rods should be used as the constitu-

ents for the thermal protection components subjected to

severe erosion of two-phase flow, and carbon rods with high

carbon fiber density should be used as the constituents for

thermal protection parts subjected to severe chemical

ablation.

Acknowledgment

This work was financially supported by the National Natural

Science Foundation of China (Contract No. 50876091).

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